Chromatography: phases and calculations
Select Lesson
Exam Board
Select an option
Chemistry practicals
Fuels and Earth science
Rates of reaction and energy changes
Groups in the periodic table
Extracting metals and equilibria
States of matter and mixtures
Key concepts
Chemical equations
Ionic equations and half equations - Higher
Hazards, risks and precautions
The history of the atom
The atom and isotopes
Calculating relative masses
Calculating relative atomic mass - Higher
The history of the periodic table
Electronic configuration
Ions and their formation
Ionic bonding: properties of compounds and naming
Covalent bonding
Giant covalent substances
Metallic bonding and alloys
Conservation of mass
Molecular and empirical formulae
Mole calculations and Avogadro's constant - Higher
Concentrations of solutions
Limiting reactants and masses - Higher
Practical skills
Explainer Video
Summary
Chromatography: phases and calculations
In a nutshell
Chromatography is one type of analytical method that can be used to separate substances. This summary will review paper chromatography, methodology and associated calculations.
Chromatography
Definition
An analytical method used to separate substances in a mixture.
Chromatography phases
A mobile phase | Molecules can move. This is a liquid or gas. |
A stationary phase | Molecules cannot move. This is a solid (or very thick liquid). |
Substances move between the mobile and stationary phases until an equilibrium is formed. Different chemicals move at different speeds between the phases, causing components in a mixture to become separated.
Example
A chemical that spends more time in the mobile phase than stationary phase will move further.
Paper chromatography
In paper chromatography, the mobile phase is the solvent (usually ethanol or water) and the stationary phase is the chromatography paper.
The amount of time chemicals spend in each phase depends on:
- How soluble they are in the solvent
- How attracted they are to the paper
Chemicals with a high solubility and less attraction to the paper spend most time in the mobile phase, hence travel further.
Separating dyes in ink
Procedure
| 1. | Draw a pencil line above the bottom of the chromatography paper. |
| 2. | Place ink spots along the line. |
| 3. | Place paper inside a beaker containing the solvent and cover with a watch glass. |
| 4. | The solvent will travel up the paper, causing separation of the ink spots. Different dyes travel at different speeds causing them to end up at different distances relative to the start line. |
| 5. | Compare the pattern of the spots to known references to identify the chemicals present. The end pattern is called a chromatogram. |
 |
|
Calculations
An value is a standard measure that compares the separation that occurs.
This is the ratio between the distance moved by the substance and the distance moved by the solvent (solvent front).
Different chemicals have different values in different solvents. This means these measurements can be used to help identify components in compounds.
Important points:
- You measure the distance moved by the substance from the pencil baseline to the centre of the spot.
- values have no units and range from zero to one.
- The further a chemical moves through a stationary phase, the larger the value.
 | 1. Solvent front 2. Baseline, where to start measurement 3. Distance travelled by molecule 4. Distance travelled by solvent |
Example
Calculate the value for a dye that moves in a solvent that moves .
The value for the dye is .
FAQs - Frequently Asked Questions
How much time do chemicals spend in each phase?
The amount of time chemicals spend in each phase depends on how soluble they are in the solvent and how attracted they are to the paper.
What is the Rf value?
The Rf value is the ratio between the distance moved by the substance and the distance moved by the solvent (solvent front).
What are the different phases in paper chromatography?
In paper chromatography, the mobile phase is the solvent (usually ethanol or water) and the stationary phase is the chromatography paper.